1. Field of the Invention
The present invention relates to a diversity receiver used in a digital radio communication appliance for digital mobile communications, digital satellite communications, digital mobile satellite communications, or the like.
2. Description of the Related Art
In digital mobile communication systems, a fading phenomenon may occur, since electromagnetic waves are reflected, diffracted, and scattered by regions and articles located around a mobile station. In the fading phenomenon, amplitudes of received signals and phases of these received signals are severely varied. Under such a fading environment, since the amplitudes of the received signals and the phases thereof are severely varied, a coherent detection using recovered carrier can be hardly realized.
As a result, a differential detection may be usually employed. In the differential detection, a differential coding operation is carried out on a transmission side, whereas a received signal before 1 symbol is used as a reference signal.
As one of the conventional techniques capable of improving the performance under such a fading environment, the diversity reception technique is known in the field, by which signals are received in plural branches, and then these received signals are combined with each other, or selected. Also, as the diversity reception techniques using the differential detection, there are the antenna selecting diversity reception, the postdetection selecting diversity reception, and the postdetection combining diversity reception. In the antenna selecting diversity reception, the demodulated result of such a branch that the received signal power is larger than other received signal power is selected every burst. In the postdetection selecting diversity reception, the demodulated result of such a branch that the received signal power is larger than other received signal power is selected every symbol. In the postdetection combining diversity reception, the differential detection results of the respective branches are combined with each other.
As one prior art, a description will now be made of an arrangement of a differential detection diversity receiver and operation of the diversity receiver in which a quadrature phase shift keying (QPSK) signal which has been differentially coded is detected, and thereafter the detected QPSK signals are combined with each other for the diversity reception.
FIG. 17 is a block diagram for showing a conventional differential detection diversity receiver. The differential detection diversity receiver is described in, for example, the publication xe2x80x9cBER Performance of QDPSK with Postdetection Diversity Reception in Mobile Radio Channelsxe2x80x9d written by F. Adachi and K. Ohono, IEEE Transactions on Vehicular Technology, Volume 40, No. 1, in 1991, pages 237 to 249. In the drawing reference numerals 11 and 12 show delay circuits, reference numerals 13 and 14 indicate multipliers, reference numeral 15 represents an adder, and reference numeral 16 shows a detector. Also, reference numerals 101 and 102 show received baseband signals, reference numerals 103 and 104 represent received baseband signals before 1 symbol, reference numerals 105 and 106 denote differential detection results, reference numeral 107 indicates soft decision data, and reference numeral 108 shows hard decision data.
Next, the operation of the differential detection diversity receiver will now be explained.
When the received baseband signal 101 of the branch 1 is inputted, the delay circuit 11 delays the received baseband signal 101 by time corresponding to 1 symbol to output the delayed received baseband signal as the received baseband signal 103 before 1 symbol.
The multiplier 13 performs the complex multiplication between the received baseband signal 101 and the received baseband signal 103 before 1 symbol to output the differential detection result 105.
When the received baseband signal 102 of the branch 2 is inputted, the delay circuit 12 delays the received baseband signal 102 by time corresponding to 1 symbol to output the delayed received baseband signal as the received baseband signal 104 before 1 symbol.
The multiplier 14 performs the complex multiplication between the received baseband signal 102 and the received baseband signal 104 before 1 symbol to output the differential detection result 106.
The adder 15 performs the complex adding operation between the differential detection result 105 of the branch 1 and the differential detection result 106 of the branch 2 to output the soft decision data 107.
The detector 16 performs the hard decision with respect to the soft decision data 107 to output the hard decision data 108.
As previously explained, the differential detection diversity receiver for executing the postdetection combining diversity reception can improve the performance by combining the differential detection results of the respective branches, as compared with the other receiver which does not execute the diversity reception. Also, the postdetection combining diversity may represent better performances than that of the antenna selection diversity, or that of the postdetection selection diversity for selecting the demodulated result of such a branch that the received signal power is larger than other received signal power every burst, or every symbol.
On the other hand, in digital mobile communication systems, one trial has been made in order to effectively utilize a frequency by reducing a zone radius of a cell and by repeatedly using the same frequency. At this time, there is such a problem that the co-channel interference will occur, which is caused by the electromagnetic waves leaked from the adjoining cells with using the same frequency, and therefore the performance is deteriorated.
As one of the techniques capable of mitigating the deterioration in the performance caused by the co-channel interference, the least-squares combining diversity technique is known. In the least-squares combining diversity, the received signals of the respective branches are combined with each other in order that the square mean value of the error signals can be reduced as small as possible.
As second prior art, an arrangement and operations of a least-squares combining diversity receiver capable of performing least-squares combining with respect to a QPSK signal will now be described.
FIG. 18 is a block diagram for indicating an arrangement of a conventional least-squares combining diversity receiver. The conventional least-squares combining diversity receiver is described in, for example, the publication xe2x80x9cInterference Cancelling Characteristics of Diversity Reception with Least-Squares Combiningxe2x80x94MMSE Characteristics and BER Performancexe2x80x9d written by H. SUZUKI, IEICE Transactions on Communications, volume J 74-B-II, No. 12 in 1991, pages 637 to 645.
In the drawing, reference numerals 21 and 22 indicate multipliers, reference numeral 23 shows an adder, reference numeral 24 indicates a detector, reference numeral 25 represents a subtracter, and reference numeral 26 shows a tap coefficient control circuit. Also, reference numerals 201 and 202 show received baseband signals, reference numerals 203 and 204 represent tap coefficients, reference numerals 205 and 206 are multiplication results, reference numeral 207 shows soft decision data, reference numeral 208 indicates hard decision data, and reference numeral 209 denotes an error signal.
Next, the operation of the least-squares combining diversity receiver will now be described.
The multiplier 21 performs the complex multiplication between the tap coefficient 203 determined by the tap coefficient control circuit 26 and the received baseband signal 201 of the branch 1 to output the multiplication result 205.
The multiplier 22 performs the complex multiplication between the tap coefficient 204 determined by the tap coefficient control circuit 26 and the received baseband signal 202 of the branch 2 to output the multiplication result 206.
The adder 23 performs the complex adding operation between the multiplication result 205 of the branch 1 and the multiplication result 206 of the branch 2 to output the soft decision data 207.
The detector 24 performs the hard decision with respect to the soft decision data 207 to output the hard decision data 208.
The subtracter 25 executes the complex subtraction between the soft decision 207 and the hard decision 208 to output the error signal 209.
When the error signal 209 is entered, the tap coefficient control circuit 26 controls the tap coefficients 203 and 204 in such a manner that the square mean value of the error signal 209 can be reduced as small as possible. As the adaptive algorithm for controlling the tap coefficient, the least mean square (LMS) algorithm or the recursive least squares (RLS) algorithm are often employed.
As previously explained, in the least-squares combining diversity receiver, even when the received baseband signals 101 and 102 contain the co-channel interference components, since the tap coefficient is controlled in such a manner that the square mean value of the error signal can be reduced as small as possible, the adverse influence caused by the co-channel interference can be mitigated.
The conventional differential detection diversity receiver and the conventional least-squares combining diversity receiver are arranged and operated in the above-described manners. The performance of the differential detection diversity receiver is deteriorated under such a condition that the co-channel interference is present. The performance of the least-squares combining diversity receiver is deteriorated in such a case that the fast variation occurs in the fading phenomenon.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide a diversity receiver capable of representing better performance even under such a condition that co-channel interference is present, and even when the fading phenomenon is quickly varied.
A diversity receiver, according to a first aspect of the invention, is featured by comprising; a differential detection diversity reception circuit for performing a diversity reception with employment of a differential detection to thereby output hard decision data thereof; a least-squares combining diversity reception circuit for performing a least-squares combining diversity reception to thereby output a differentially decoded result of the hard decision data; a selection control circuit for producing a selection control signal used to select one of the hard decision data outputted from the differential detection diversity reception circuit and the differentially decoded result outputted from the least-squares combining diversity reception circuit based upon a predetermined selection basis; and a selection circuit for selecting one of the hard decision data and the differentially decoded result based upon the selection control signal.
A diversity receiver, according to a second aspect of the invention, is featured by that the differential detection diversity reception circuit further outputs soft decision data; the least-squares combining diversity reception circuit further outputs reliability information; the selection control circuit produces a selection control signal used to select one of the soft decision data/hard decision data outputted from the differential detection diversity reception circuit, and the reliability information/differentially decoded result outputted from the least-squares combining diversity reception circuit based upon a predetermined selection basis; and the selection circuit selects one of the soft decision data/hard decision data and also the reliability information/differentially decoded result in response to the selection control signal.
A diversity receiver, according to a third aspect of the invention, is featured by that while receiving the hard decision data outputted from the differential detection diversity reception circuit, the differentially decoded result outputted from the least-squares combining diversity reception circuit, and further a prerecognized reference signal, the selection control circuit calculates a correlation among the reference signal and a portion of the hard decision data, corresponding to the reference signal; a portion of the differentially decoded result, corresponding to the reference signal; and the selection control circuit produces a selection control signal used to select one of the soft decision data/hard decision data outputted from the differential detection diversity reception circuit and also the reliability information/differentially decoded result outputted from the least-squares combining diversity reception circuit based on a dimension of coincident symbol numbers.
A diversity receiver, according to a fourth aspect of the invention, is featured by that the differential detection diversity reception circuit further produces an error signal; the least-squares combining diversity reception circuit further outputs an error signal; and the selection control circuit produces a selection control signal used to select one of the soft decision data/hard decision data outputted from the differential detection diversity reception circuit and the reliability information/differentially decoded result outputted from the least-squares combining diversity reception circuit based upon such a fact as to whether or not a mean squared value of the error signal outputted from the differential detection diversity reception circuit is larger than a mean squared value of the error signal outputted from the least-squares combining diversity reception circuit.
A diversity receiver, according to a fifth aspect of the invention, is featured by that the selection circuit produces a selection control signal used to select one of the soft decision data/hard decision data outputted from the differential detection diversity reception circuit and the reliability information/differentially decoded result outputted from the least-squares combining diversity reception circuit based upon a fading pitch detected from the received signals of the plural branches.
A diversity receiver, according to a sixth aspect of the invention, is featured by that the selection control circuit produces a selection control signal used to select one of the soft decision data/hard decision data outputted from the differential detection diversity reception circuit and the reliability information/differentially decoded result outputted from the least-squares combining diversity reception circuit based upon received signal power calculated from the received signals of the plural branches.